The low heat build-up and light weighting of a tire has been recently designed from social strong request for low fuel cost. And, among tire members, the light weighting of an inner liner has been also carried out, which is provided in the inside of a tire and which has functions of reducing air leak quantity (air permeation quantity) from the inside of a pneumatic tire to the outside and improving air retention property.
At present, as a rubber composition for an inner liner, the improvement of the air retention property of a tire is carried out by highly compounding a butyl rubber. However, the butyl rubber is superior in the lowering effect of air permeation quantity but since sulfur is hardly dissolved in the butyl rubber, there have been problems that crosslinking density is low and adequate strength is not obtained. Consequently, for example, as described in Japanese Unexamined Patent Application Publication No. 2006-328193, crack growth resistance has been improved by compounding a butadiene rubber in a rubber composition for an inner liner including mica, as a rubber component in addition to a butyl rubber, a natural rubber or an isoprene rubber. However, there has been a problem that when the compounding ratio of a butadiene rubber is increased, the air permeation quantity is increased.
Further, there has been a problem that when the compounding ratio of a natural rubber is increased for improving the low fuel cost of a vehicle, the air permeation quantity is also increased.
Thus, it has been difficult that all properties such as air permeation resistance, low heat build-up property and strength at break are improved in the rubber composition for an inner liner.